Design a Phone Directory which supports the following operations:

get: Provide a number which is not assigned to anyone. check: Check if a number is available or not. release: Recycle or release a number.

Example:

// Init a phone directory containing a total of 3 numbers: 0, 1, and 2.
PhoneDirectory directory = new PhoneDirectory(3);

// It can return any available phone number. Here we assume it returns 0.
directory.get();

// Assume it returns 1.
directory.get();

// The number 2 is available, so return true.
directory.check(2);

// It returns 2, the only number that is left.
directory.get();

// The number 2 is no longer available, so return false.
directory.check(2);

// Release number 2 back to the pool.
directory.release(2);

// Number 2 is available again, return true.
directory.check(2);

Solution

public class PhoneDirectory {
    int max = 0;

    boolean occupy[];
    int cur = 0;
    int cnt = 0;

    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        occupy = new boolean[maxNumbers];
        max = maxNumbers;
    }

    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    public int get() {
        if(cnt == max) return -1; // if no available space
        for(int i = cur; i < cur + max; i++) { // start from cur, wrap around
            if(!occupy[i % max]) {
                occupy[i % max] = true;
                cur = i + 1;
                cnt++;
                return i % max;
            }
        }
        return -1;
    }

    /** Check if a number is available or not. */
    public boolean check(int number) {
        return !occupy[number];
    }

    /** Recycle or release a number. */
    public void release(int number) {
        if(occupy[number]) {
            occupy[number] = false;
            cnt--;
        }
    }
}

/**
 * Your PhoneDirectory object will be instantiated and called as such:
 * PhoneDirectory obj = new PhoneDirectory(maxNumbers);
 * int param_1 = obj.get();
 * boolean param_2 = obj.check(number);
 * obj.release(number);
 */